1. Field of the Invention
The present invention relates to a simulation method for an electronic circuit, and more specifically to a circuit simulation method for detecting an error in values of inductance and mutual inductance in a circuit composed of linear circuit elements.
2. Description of Related Art
In the prior art, a circuit simulator typified by "SPICE" (Simulation Program with Integrated Circuit Emphasis) has been used in order to analyze electric characteristics of an electronic circuit. In this circuit simulator, as shown in FIG. 1, circuit information 21 composed of a circuit structure 22 including interconnection relations between circuit elements and circuit element characteristics 23 including resistance values, mutual inductance values, model parameters of transistors, etc., are inputted to a circuit simulation unit 20, which has a function of analyzing DC characteristics, switching characteristics, etc. of the circuit. The circuit simulation unit 20 outputs the result of the analysis to a circuit characteristics file 24. Based on the result of the analysis, a circuit designer confirms the result of his design and makes selection of a draft design.
In the case of analyzing the circuit characteristics by use of the circuit simulator such as SPICE, when the parameters of the circuit to be analyzed include errors, a correct analysis cannot be often carried out. In particular, when the element characteristics of the mutual inductance is erroneous, a linear circuit element network which should be inherently passive, loses passivity in some situation. In this case, node voltage causes oscillation, so that the simulation can be no longer carried out. In general, in order to check the passivity of this type circuit, it is sufficient if examination is made as to whether or not all real-number parts in eigenvalues of a circuit matrix are positive definites.
As a second prior art, Japanese Patent Application Pre-examination Publication No. JP-A-07-129637 (an English abstract of which is available from the Japanese Patent Office, and the content of the English abstract of JP-A-07-129637 is incorporated by reference in its entirety into this application) discloses a method for displaying the eigenvalues of the circuit matrix by use of a transient analysis of the circuit simulator.
FIG. 2 is a flowchart illustrating this second prior art. In a step S21, circuit information is input, and in a step S22, a time Tcal for calculating the eigenvalue is read. In succeeding steps S23 and S24, a circuit matrix is prepared, and a DC analysis is carried out. In a step S25, a transient analysis is conducted, and when the analysis time becomes the time Tcal, matrix data is written into a magnetic disk. After a series of steps (S31 to S36) for the transient analysis are completed, a step S26 for processing the eigenvalue display is conducted. In brief, the matrix data is read out (step S37), and the eigenvalue is calculated on the basis of the read-out matrix data (step S38). A real-number part and an imaginary-number part of the calculated eigenvalue are displayed on an X coordinate axis and a Y coordinate axis, respectively (step S39).
However, the following problems have been encountered in the prior art example shown in FIG. 2. First, since the data is prepared by use of the circuit simulator, the circuit simulation is necessarily executed without exception, and therefore, before execution of the circuit simulation, it is not possible to check whether or not the circuit concerned is passive. Secondly, in order to obtain the eigenvalue from the circuit matrix, a calculation needs a large time when the number of elements, namely, items, in the matrix are large. This approach is not suitable when the scale of the circuit to be analyzed is large.
In the prior art example shown in FIG. 2, furthermore, it is possible to discriminate that the circuit is not passive, by visually observing the display coordinate on which the eigenvalues are plotted. However, it is not possible to discriminate what is a cause for non-passivity of the circuit, namely, which of circuit elements included in the circuit has a problem. As a result, in order to locate a cause for the non-passivity, it is necessary to repeat such an trial-and-error operation that the circuit information is changed and the eigenvalues at that time are checked.